Most human primary and permanent cell lines grown in vitro require the presence of serum, either human or of other animal origin, for stable cell growth and proliferation. Serum is the supernatant produced by clotting of blood and spinning off the sediment. Attempts to culture serum-dependent cell lines in culture media, in which serum is substituted with chemical additives or with platelet-free plasma, usually fail. In contrast, platelet lysate can in some culture systems be used to support cell proliferation. This is in part due to the presence of platelet derived growth factors (PDGF), which are released during platelet activation that occurs during blood clotting. This provides the growth-promoting serum with its proliferation-stimulating properties, which are essential for the proliferation and growth of the majority of mammalian cell lines in vitro.
It can be assumed that cells in vivo have a similar dependence on PDGF, as cells in vitro. However, serum is not present in blood circulation in vivo. It is not compatible with the physiological requirements for survival of higher organisms. Growth factors are consequently present in vivo in the blood not as free substances, rather in vesicles within platelets. PDGF is however released from platelets upon activation.
As growth factors are essential for the growth of normal and malignant cells, they must be made available to the cells. Cells that have left the G0 phase and are poised in the G1 phase, require appropriate signals regarding entry into S phase and associated cell proliferation. Therefore, activation of platelets, resulting in the release of growth factors, is an important mechanism inherent in the initiation of cell proliferation and cell growth in vivo. Another aspect of this mechanism is the topology. The growth factors have to be released from the platelets at the site where they are needed. The platelet activation must take place exactly at the site of the cells committed to growth. The mechanism accomplishing this has until now not been subject to in depth investigation.
It is known in the art that platelets bind to the surface of some tumor cells and that this binding is involved in tumor metastasis. For example, Modery-Pawlowski et al. (Biomacromolecules 2013, 14, 910-919) and Takagi et al. (PLOS ONE, August 2013, 8, 8, e73609) disclose that a physical interaction between platelets and tumor cells can play an important role in the metastasis of these cells.
In some tumor cells, platelet receptors are overexpressed, resulting in enhanced platelet aggregation to the surface of tumor cells. Although the studies described previously disclose a correlation between platelet-cell surface-binding with the metastasis of the cells, a relationship between cell growth, in particular tumor cell proliferation, and a physical interaction of platelets on the cell surface has not been described.
Suppression of cell proliferation plays a central role in the treatment of diseases associated with undesired cell growth. Physicians and manufacturers of pharmaceutical products have attempted for decades to develop effective strategies to inhibit undesired cell proliferation of a tumor. The provision of PDGF to tumor cells via an interaction with platelets can be considered as an important target to inhibit the growth or proliferation of tumor cells.
Glycosaminoglycans comprise a linear structure of repeating disaccharides. Heparin is one example of the glycosaminoglycans. Heparin is an endogenous polysaccharide, which has an inhibitory effect on the coagulation cascade and is therefore used therapeutically for anticoagulation. Heparin is known not only as an anticoagulant, but also as a potential therapeutic agent for the prevention of tumor metastasis. The use of heparin for the inhibition of tumor cell metastasis has been described in the art (Ma et al, Invest New Drugs (2012) 30: 508). The use of low molecular weight heparin resulted in reduced metastasis of colon tumors. Activity against cell growth or cell proliferation of tumors after administration of low molecular weight heparin was not identified by the authors of this study.